There are many well known varieties of data transmission systems in which analog-to-digital (A/D) conversion techniques are employed in combination with fiber-optic communications links. Still, the need frequently arises to develop unique transmission systems and related components and sub-circuits in order to satisfy the particular demands of new systems applications. The present invention was conceived and developed to satisfy one class of such a demanding systems application. Accordingly, the preferred embodiment of the invention will be described herein with reference to that particular systems application, but it will be recognized that the novel system circuit of the invention can be readily used to satisfy the needs of other systems applications in the same class of high speed, long distance, data transmission multiplexing circuits.
A preferred embodiment of the invention was developed for application in the Main Control Room (MCR) of the Alternating Gradient Synchrotron (AGS) that is in operation at Brookhaven National Laboratory, Upton, NY. In that application there existed a need to select and display analog signals from a plurality of devices that are widely spaced around the AGS Particle Accelerator Ring. These selected signals have to be transmitted for relatively long distances (e.g. up to 3 Km (Kilometers)), and have to be screened for selection as desired by a human operator, and then the signals need to be continuously displayed, in real time, on a number of control monitors in the MCR. A subset of those signals, that may originate from as many as 250 possible analog signal generating devices has to be selected and simultaneously displayed. The analog signals of interest have a bandwidth from DC (direct current) level to 100 Khz (Kilohertz), in the range of +/-10 V (Volts).
In this AGS systems application there exists a system of analog signal distribution and multiplexing, which consists mainly of a combination of electronic and mechanical switches that are operable to select analog signals at remotely positioned source devices, and various operational amplifiers and line drivers that are used to distribute these analog format signals to multiple monitoring consoles at a centralized monitoring point in the Main Control Room. This existing system had many disadvantages, some of which were that the signals were transmitted in analog form, and the selection of the signals to be multiplexed and monitored was complex and difficult to operate.
In order to solve the unique problems presented by this demanding systems application, a new digital data distribution system needed to be conceived and developed. This new system, which comprises the subject invention, can be regarded as a one way digital data communication system for effecting high speed, long distance data multiplexing transmission from widely spaced signal generating source devices in the field, e.g. the source devices spaced around the AGS Particle Accelerator Ring, to the display consoles in the MCR.
There are two well known notable advantages of digital transmission, versus analog transmission, that make it extremely attractive for use in such high speed, long distance, multiplexed data communication systems. They are:
a) Noise does not accumulate at repeaters and thus such noise becomes a secondary consideration in system design, whereas in analog systems it is the primary consideration.
b) The digital format lends itself ideally to solid-state technology and to computer control and data processing.
While digital data transmission systems have increasingly used modems to transfer signals over the analog telephone network, the existing telephone networks have been turning to the use of digital techniques to transfer analog signals.
In order to achieve the desired accuracy of transmission of analog monitored signals in the range of +/-10 V, -10 V, it has been determined that A/D conversion has to ensure 12 bits precision. In the preferred embodiment of the invention, as used in the application at the AGS, a minimum of ten samples per period are requested for the digitalized signals. This led to a sampling rate of 1 Mhz (Megahertz), with 12 bits accuracy per sample.
The prior-art technologies of digital telephony, and lately of the Integrated Services Digital Network (ISDN) technology, does not satisfy the unique and demanding systems and bandwidth requirements of the analog signals that need to be monitored and transmitted at high speeds, over long distances, and be multiplexed in the environment of the AGS application. The high speed, high accuracy data acquisition systems now commercially available do not have the flexibility to multiplex and select as many channels as are required by this demanding type of systems application. Other technologies were investigated but it was found that none of them possess the needed high-level capability of continuous, real time data acquisition and needed transmission capabilities for long distances. Accordingly, it was determined that the known, existing technologies investigated could not afford appropriate technological or suitable cost/performance solutions for the specific demanding systems requirements of the AGS Particle Accelerator Ring and MCR.
The design of a new high speed, long distance, multiplexing digital data distribution system, according to the present invention, has the following main features. It uses analog input signals in the frequency range from direct current (DC), or zero hertz level, to 100 Khz. These analog signals are sampled at a minimum rate of ten samples per cycle in the preferred embodiment, but it will be understood that different sampling rates can be used when applying the invention to less demanding transmission systems applications in the long distance, high speed, multiplexing class. The sampling rate used in the preferred embodiment is 1 Mhz and the data accuracy is 12 bits per sample, for a voltage range of +/-10 V. The sampled analog input signals are digital encoded and framed using a suitable Pulse Code Modulation (PCM).